In modern digital cameras, including smartphone cameras, it is common to provide a flash that uses a pcLED. A common flash is a GaN-based blue LED die mounted in a round reflective cup on a rigid printed circuit board. A layer of YAG phosphor (emits yellow-green) fills the cup. Since the LED die is very thin, almost all light is emitted from the top surface of the die. A circular Fresnel lens is then positioned over the cup to create a generally conical light emission pattern (having a circular cross-section) to illuminate a subject for the photograph. A cover plate, forming part of the camera body, typically has a circular opening for the lens.
Since the field of view of the camera is rectangular, much of the light emitted from the flash, having a circular cross-section, illuminates areas surrounding the subject and is wasted. Such unnecessary illumination may also be bothersome to those not in the picture.
Further, due to the shape of the cup and the phosphor in the cup, the phosphor is not uniform over the LED die, resulting in color non-uniformity vs. angle.
Further, due to the use of the rigid printed circuit board, the thinness of the flash module is limited.
Further, the lens must be spaced away from the top surface of the LED die by a certain minimum distance (e.g., the focal length) in order to properly redirect the light. This minimum distance significantly adds to the thickness of the flash module.
Further, there is substantial back-reflection from the lens back toward the cup and LED die.
Further, the bottom inner edge of the reflector cup facing the sides of the LED die has a thickness that is typically greater than the height of the LED semiconductor layers, so the inner edge of the cup blocks the side light or reflects it back into the LED die.
Further, since almost all light is emitted from the top surface of the LED die, the reflective cup has limited usefulness in shaping the beam, and the resulting beam is not very uniform across the field of view of the camera.
Further, since almost all light is emitted from the top surface of the LED die in a Lambertian pattern, the reflector cup has to have relative high walls to redirect and collimate the “angled” light emitted from the LED die. Any light rays that are not reflected (collimated) spread out at wide angles. The high walls of the reflector limit the minimum thickness of the flash module.
It is known to affix a lens over the LED die for a flash, where the lens has a cavity for the LED die, such as described in patent publication KR2012079665A. The lens has a rectangular top surface and curved side surfaces. However, a significant portion of the light escapes from the sides and is not reflected toward the subject. Also, the prior art lens is relatively thick, resulting in a thick flash module.
What is needed is a thin LED flash module for a camera that more uniformly and efficiently illuminates a subject.